U.S. patent number 4,225,858 [Application Number 05/850,270] was granted by the patent office on 1980-09-30 for doppler intrusion detector with dual phase processing.
This patent grant is currently assigned to I.E.I. Proprietary Limited. Invention is credited to Martin T. Cole, Graeme R. Strahan.
United States Patent |
4,225,858 |
Cole , et al. |
September 30, 1980 |
Doppler intrusion detector with dual phase processing
Abstract
The reflected signal of a microwave or ultrasonic Doppler
intrusion detector is split into two different channels, each
containing a mixer for hetrodyning the received signal with a
reference signal. The reference signal for one mixer is the
transmitted signal, while the reference signal for the other mixer
is the transmitted signal with a phase shift. Thus the outputs of
the mixers are of the same frequency but different phase. A phase
comparator produces two mutually exclusive pulse trains as a
function of the outputs of the two mixers, one said output
representing motion toward the detector and the other representing
motion away from the detector. These pulse trains are applied
through a bidirectional counter to upper and lower limit sensors.
An alarm circuit is activated if either limit is exceeded, thus
providing an alarm in response to sufficient motion in either
direction.
Inventors: |
Cole; Martin T. (East
Bentleigh, AU), Strahan; Graeme R. (Upwey,
AU) |
Assignee: |
I.E.I. Proprietary Limited
(Victoria, AU)
|
Family
ID: |
3698731 |
Appl.
No.: |
05/850,270 |
Filed: |
November 10, 1977 |
Foreign Application Priority Data
Current U.S.
Class: |
340/554; 367/94;
342/28 |
Current CPC
Class: |
G01S
13/56 (20130101); G08B 13/1627 (20130101) |
Current International
Class: |
G01S
13/00 (20060101); G01S 13/56 (20060101); G08B
13/16 (20060101); G08B 013/18 () |
Field of
Search: |
;340/560,554
;343/5PD |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Swann, III; Glen R.
Attorney, Agent or Firm: Learman & McCulloch
Claims
We claim:
1. In an intrusion detector in which a signal generated by a
microwave or ultrasonic generator is radiated into an area under
surveillance and reflected from every object in the area; a
receiver to receive said reflected signal and to present said
reflected signal to a pair of mixers, each said mixer mixing said
reflected signal with a portion of the transmitter signal, the
improvement comprising (a) splitting of said portion of said
transmitter signal into two paths, and (b) introducing a relative
phase difference between said two paths, the resultant signals
being fed into said mixers to produce Doppler signals of equal
frequency and amplitude but differing phase, said Doppler signals
being presented to phase detecting means having two output channels
controlled such that only one such channel may produce an output
pulse at any instant, the channel outputs representing incremental
and decremental movement of said objects in said area
respectively.
2. The invention of claim 1 wherein said phase detecting means
includes a set of simple logic gates for providing at least a twin
channel output determined by the relative phase difference in said
Doppler signals such that if one signal leads another in phase then
one output transmits the signal and vice versa.
3. The invention of claim 1 including a counter for receiving
pulses from both channels, the arrangement being such that counters
will count incrementally all pulses from one of the channels and
will count decrementally all pulses for the other of the channels
so that the counter will count either incrementally or
decrementally responsive to movement in the appropriate direction,
thus resulting in integration of unidirectional movement in the
surveyed area.
4. The invention of claim 3 wherein the counter is connected to an
alarm device which alarm device is actuated upon said counter
counting a predetermined number of incremental or decremental
pulses, said number being proportional to the distance of
unidirectional movement of an object in the area surveyed.
5. The invention according to claim 3 in which the value of the
count which has occurred after movement has occurred will slowly
decay to zero in order to prevent any eventual false alarm caused
by occasional interference, building movement, or other
environmental factors and willful defeat yet maintaining high
sensitivity.
6. The invention according to claim 3 in which the sensitivity of
the detector is essentially independent of the range of the object
being detected and independent of the setting of any range control
device thereby permitting the use of realistic sensitivity control
to allow adjustment of the sensitivity of the detector achieving
the best compromise between false alarm immunity and ability to
detect intrusion within a particular area and environment.
7. The invention of claim 3 wherein said output channels of said
phase detection means produce symmetrical outputs of equal strength
irrespective of the amplitude of said reflected signals and said
Doppler signals presented to said phase detecting means, such that
said counter will give equal weight to each pulse from either
channel thereby preventing possible false activations of the
detector near the extremity of the range of detection of said
device.
8. The invention of claim 1 including a preamplifier for amplifying
the received signal prior to presentation to the mixers said
preamplifier including a gain adjustment to allow different size
areas to be placed under surveillance, and automatic gain control
circuitry effective upon the reflected signal to prevent loss of
information concerning the direction of motion due to large signal
reflection from objects close to the detector which may otherwise
overload the preamplifier or mixers.
Description
BACKGROUND OF THE INVENTION
The invention relates to a movement detector utilizing the
transmission and reception of signals for the purpose of detecting
moving objects in a manner which effectively discrimates against
unwanted interfering phenomena. These phenomena could otherwise
cause false operation of the detector, or could prevent the
operation of the detector. The signals may be produced from a micro
wave or ultrasonic generator. A micro wave detector is described in
our earlier application no. USSN 637,394 filed Dec. 3, 1975.
Movement detectors are known for detecting the movement of objects
(preferably human) within an area of coverage. The detectors may
operate with micro wave generators or with ultrasonic acoustic
generators.
In general, an ultrasonic acoustic signal is transmitted into the
area under surveillance and the signals reflected back from every
object in the area, are returned to a receiver. Should one of the
objects in that zone be moving then the signal received after
reflection off that object will be of changing phase relative to
the transmitted signal. The rate of change of phase is proportional
to the velocity of movement of the object. By mixing this received
signal with a portion of the transmitted signal, there is produced
a resultant frequency equal to this rate of change of phase. This
frequency is amplified and processed in order to activate an alarm
when movement of objects within the area occurs.
Refinement of such designs in the form of controlled amplifier
bandwith, threshold level and counting means reduces the
possibility of false activation caused by objects moving in a
manner not characteristic of human movement. These characteristics
are speed, duration and frequency of occurrence.
The disadvantage of such design is that there is no ability to
discriminate between linear and reciprocal motion. Thus, vibration
could cause a false activation. A further improvement of such
detectors has been the ability to determine the direction of motion
taking place. By summing algebraeically the motion, then vibration
shows zero net progression through the area, whilst motion of a
human would show a nonzero net progression. Sufficient net
progression would cause activation of the detector.
Of fundamental importance to the correct operation of such improved
detectors is means to ensure perfect balance in detection of motion
in one direction versus the other.
If the detector is more sensitive to motion in one direction than
the other, then vibration would not appear as a perfectly zero net
progression, such that false activation would result.
This invention achieves this balance by providing an improved
method of obtaining and processing the information pertaining to
the magnitude, velocity and direction of motion of the object in
such a manner that errors are overcome and false activation
prevented.
Existing designs utilize a mixer circuit to obtain two signals in
phase quadrature. These two signals are of the same frequency and
arise from the Doppler effect. The phase difference of the two
frequencies will be such that if motion in the area occurs in one
direction, then one signal will phase lead the other. If the
direction of motion reverses, then that signal will phase lag the
other. This relative phase lead or lag is used to determine the
direction of motion.
These two Doppler signals are achieved by mixing the received
ultrasonic signal with a portion of the transmitted signal.
90.degree. phase quadrature is achieved by dividing the received
signal into two paths (after amplification) and by inserting a
45.degree. phase lead circuit into one path, and a 45.degree. phase
lag circuit into the other path. Upon mixing each of these signals
with the same transmitter - derived signal, there results the said
two Doppler signals in phase quadrature. A disadvantage of this
system is that the phase lead and lag circuit cause attenuation and
are frequency sensitive. Because of the Doppler shift the frequency
of the received signal is effectively higher of lower than the
transmitted frequency when movements exists. Because the phase lead
or lag circuitry will only produce a 45.degree. phase shift at one
particular frequency, then errors in phase shift will occur at the
Doppler shifted frequency. These errors could cause difficulties in
the subsequent signal processing circuitry.
Therefore, embodied is this invention in the improvement whereby no
phase change is introduced into either received signal path.
Instead, a phase difference is introduced to the
transmitter-derived signal, because this signal is always of
constant frequency. Accordingly both the receiver signal and the
transmitter signal are split into two paths. A phase delay is
introduced into one of the transmitter signal paths. Two mixers are
provided, resulting in two Doppler signals of equal frequency but
differing phase as required.
These two Doppler signals are amplified within appropriate
bandwidth and are presented to signal processing circuitry.
The signal processing circuitry can take many forms but all
existing types known to the inventor are wired such that one signal
is deemed a "reference", with which the other "variable" signal is
compared. In such an arrangement, the reference signal is
amplified, squared, clipped, differentiated or otherwise modified,
whereas the variable signal is amplified linearly and not subject
to such degree of modification. This unequal treatment of the two
signals can give rise to problems of imbalance. The invention
includes the improvement whereby the balance of the signals is
perfectly maintained throughout. Differences in amplifier gain at
different frequencies could cause false activation of the detector,
particularly when movement occurs near the extremities of the area
of coverage, where amplifier gain mismatch is most apparent.
Accordingly the signal processing circuitry of the invention
incorporates means to reject all imbalance caused by differences in
signal amplitude.
There is provided, according to the invention a phase detecting
means for use in an intrusion detector in which two Doppler signals
of differing phase are produced by a signal generator, said phase
detecting means including a set of logic gates for providing at
least a twin channel output determined by the relative phase
difference in said Doppler signals whereby if one signal leads
another in phase then one output transmits the signal and vise
versa.
The signal processing circuitry within the device of the invention
provides a pulse train on one of two output channels. Only one such
channel may produce an output pulse at the one instant. These
channels are presented to a counter such that when an output pulse
is received from one channel, the counter will count incrementally,
whereas if an output pulse is received from the other channel, the
counter will count decrementally. The two output channels represent
motion in the forward direction and the reverse direction
respectively, so that the counter will count either incrementally
or decrementally in response to movement in the appropriate
direction. Sufficient motion in one direction will result in the
counter reaching a preset limit of counting, whereupon the detector
output will be activated. Appropriately there are two such counting
limits corresponding to a degree of movement in either one
direction or the other direction. This degree of movement is
adjustable by means of a "sensitivity" control.
An important result of the achievement of balance in the invention
is the ability of the counter to effectively integrate the
progression of an object through the area. Thus movement such as a
combination of reciprocal and progressive motion will not confuse
the detector. Motion such as an intruder walking one step backwards
for each two steps forward would result in the counter registering
a net change of one step forward. Some previous designs have
attempted to overcome the confustion of the detector caused by a
combination of reciprocal and progressive motion, by the
incorporation of automatic gain control or feedback circuitry
operating on the Doppler signals. However whilst this reduces the
likelihood of false activation, it increases the chance of a person
passing undetected by means of creating reciprocal
interference.
A further important feature of the invention is that the degree of
motion required before the detector output is activated (i.e. the
sensitivity), is constant throughout the area and is independant of
the direction of motion or the setting of the range control. Where
this not the case, then the inclusion of a sensitivity control
would be pointless, because its setting would have different
effects at different points within the area. More importantly,
currently available designs in which the sensitivity is dependant
on the distance of the object are subject to false activations.
With such designs, the sensitivity actually reduces while walking
away yet increases while walking towards the detector. This
imbalance can cause a false activation even although the movement
is reciprocal about a static mean point.
Another feature of the counting system used within the invention is
the fact that an alarm activation will result from movement of a
set distance. Some other systems integrate the time for which the
movement takes place. Of greater importance is the actual degree of
movement taking place rather than the time taken to travel that
distance. Because the counter is responsive to individual pulses
which result from movement of an object through distance increments
of one half-wavelength of the ultrasonic transmission, then by
counting these pulses the counter is actually measuring the
distance travelled, in units of half a wavelength. Thus, should an
object move back at a different speed from that when moving
forward, the counter will not be confused as to the actual distance
travelled. Indeed, it would be possible to defeat a time
integrating system by (for example) walking one step back at one
speed, then two steps forward at double the speed, so that the time
taken for either movement was the same; preventing recognition of
the net progress through the area.
The counter used within the invention contains means for "decaying"
the count value stored, over a long time period. This overcomes the
possibility of false activation caused by long term building or
environmental changes. If this decay is too rapid then it is
possible for a person to pass undetected by means of walking one
step, waiting and walking another step for example. Some designs
incorporate a timer, to completely reset the counter every few
seconds, reducing the chance of a false activation but increasing
further the chance of a person passing undetected. These rapid
decay designs make the detector prone to defeat. The balance within
the invention permits decay over a longer period without prompting
false activations, thereby providing an overall higher degree of
immunity to both defeat and false activation.
The counter limit circuitry connects to the detector output
circuitry which normally incorporates a relay to interface to an
alarm system.
Incorporated within the invention is also an improved preamplifier
which amplifies the received signal and presents it to the mixers.
This preamplifier incorporates a gain adjustment configured as a
"range" control to permit adjustment of the detector to suit areas
of differing size. It has been discovered that if the preamplifier
is permitted to saturate as a result of strong reflected signals,
then essential information on the received signal is lost. Such a
situation can occur if a large object is placed close to the front
of the detector, thereby blinding the detector to movement
occurring beyond the large object. Indeed it may be possible to
confuse the direction of motion occurring beyond the large object,
resulting in false activation. Accordingly there is included in the
invention an automatic gain control circuit operating on the
ultrasonic signal (not the Doppler signals). This simply prevents
the amplitude of the signals reaching saturation, without
introducing distortion and thereby losing information. This
improvement does not permit a person to pass undetected by means of
creating reciprocal interference, yet it reduces the possibility of
false activations.
Apart from the benefits of improved detection plus rejection of
false activation, the circuitry of the invention is simplified over
previous designs and permits savings in cost and assembly.
Particularly the need for special set-up of balancing controls is
not required because the circuitry is immune to production
tolerance variations. The only adjustments are those of "range" and
"sensitivity" which are provided only for customer convenience.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of the detector apparatus.
FIG. 2 is a complete block schematic of the detector.
FIG. 3 is a schematic view of an electronic phase comparator.
FIG. 4 is an electronic counter.
FIG. 5 illustrates input and output waveforms for the phase
comparator of FIG. 3.
FIG. 6 illustrates the interrelationship of channel pulse trains
processed by the counter circuit of FIG. 4.
DESCRIPTION OF THE CIRCUITRY
Referring to FIGS. 1 and 2 ultrasonic sound waves are transmitted
via the transducer "Tx" to flood the area under the surveillance
with said sound. Ultrasonic sound waves reflected off every object
in the area are received by a second transducer "Rx". The resultant
signal is amplified by the preamplifier section (PREAMP)
incorporating two integrated circuit amplifiers (IC1a, IC1b). Also
incorporated in this section is a range control (R12), an impedance
buffer (Q2) and an automatic gain control circuit (Q1) (A.G.C.).
The next section is the mixer circuit whereby the ultrasonic signal
is split into two paths (R18, R22) and each is chopped by a
transistor (Q3, Q4). These transistors are driven from the
oscillator circuit (Q12) but with differing phase. The output from
this mixer section is two audio-frequency signals which differ only
in phase. These signals are each amplified (IC2a, IC2b) (AMP) and
presented to a phase comparator (IC3a, IC3b) which contains slight
switching hysteresis.
The resultant digital signals, indentical in all aspects except
phase, are presented to the digital phase comparator section (IC4,
IC5). The output from this section is presented to the counter
section (Q5, Q6) which incorporates a sensitivity control
(R54).
The effective value of the counter output is sensed by a pair of
threshold level switches (IC6a, IC6b). The output of these
threshold devices is presented to a configuration of transistors
(Q7, Q8, Q9) which can cause a relay (RLY) to de-energise in the
event of an alarm.
Provision is also made (Q10) to de-energise the relay in the event
that the oscillator signal disappears or the transmitting
transducer (Tx) is shorted. In addition there is a solid-state lamp
which will light during the alarm condition (Q11, D16). To further
assist with installation adjustment, there is provided a pair of
lamps, one of which will flash depending on the direction of motion
of an intruder (D9, D10).
The whole system is powered from 12 volts D.C. and this voltage is
regulated and filtered (IC7).
The transmit transducer Tx is driven by an oscillator. The receive
transducer is connected to a narrow-band preamplifier incorporating
a range control and has an automatic gain control (A.G.C.) to
prevent amplitude saturation. The received and amplified ultrasonic
signal is split into two paths and presented to two identical
mixers. One mixer is driven by the transmitter oscillator direct
whereas the other mixer is driven by the oscillator via a phase
shifting network. The output of each mixer is presented to separate
amplifiers of controlled bandwith and thence to a phase comparator.
The phase comparator incorporates a voltage threshold comparator
for each signal and a phase sensitive detector of a type depicted
in FIG. 3. The outputs of the phase comparator connect to a counter
which can count bidirectionally and is of a type depicted in FIG.
4. Upper limit and lower limit threshold voltage comparators are
connected to the counter and in turn, these couple to an alarm
output circuit which will cause an alarm should one of the counter
limits be reached.
DESCRIPTION OF PHASE COMPARATOR
In the preferred embodiment of the phase comparator, there is
included a set of NAND logic gates wired as shown in FIG. 3. The
two input channels "A" and "B" are preceded by a voltage comparator
to square the sinusoidal output signals of the amplifiers, to suit
the logic switching operation of the gates. When movement exists in
the area under surveillance, the resultant square wave signals
presented to input A and B differ in phase by 90.degree. for best
results.
FIG. 5 also shows waveforms for input and output channels of the
phase comparator. The operation is such that the first wave front
to rise will determine which output channel ("C" or "D") will
operate. Illustrated is the operation resulting from the A channel
rising before the B channel. This inhibits the operation of the C
output and permits an output pulse from the D channel. This output
pulse is duly presented to the counter.
Should motion within the area change direction then the "B" channel
wave front will precede the A channel. The symmetry of the phase
detector circuit is such that the D channel would now be inhibited
and the C channel would provide an output pulse. Thus there is
provided means for providing pulses from one channel or another,
dependant upon the phase relationship of the input channels.
An additional feature of the phase comparator is the immunity to
problems caused by possible impalance in output from the
amplifiers, caused by differing gain at various frequencies. At the
furtherest extent of the range of area coverage, one channel may
produce a waveform of narrower width than the other. This is
illustrated in two situations. Where the A channel is normal and
the B channel is reduced (B') then the output channel (D') pulse
will reduce in width. Alternatively, if the B channel is normal and
the A channel is reduced (A') then the output channel (D") pulse is
reduced to the same width. Thus, the difference in input pulse
width does not reduce the ability of the phase detector to activate
the correct output channel, but the output pulse width reduces. The
symmetry of the phase detector is such that if one amplifier gain
exceeds the other, then regardless of the direction of motion
within the area, the correct output will be activated and the
output pulse widths will be the same (although reduced) for either
channel. Any subsequent counting system responsive to pulse width
will not therefore give undue weight to the output of one channel
over the other. This overcomes false activations which would
otherwise be caused by an object moving equally forward and back,
near the extremity of the range.
In the extreme, where no output exists from one of the amplifiers,
then no output will result from either channel of the phase
detector.
The above advantages are achieved without the necessity for any
form of balancing adjustment within the amplifiers, the phase
detector or the counter. This fact not only provides for simplicity
in manufacture and set-up, but overcomes the problem of drift in
component parameters over a period of time in service.
The overall performance of the phase comparator is improved further
by the provision of slight hysteresis in the switching of the
voltage comparators.
DESCRIPTION OF THE COUNTER
In the preferred embodiment of the counter referring to FIG. 4
there is provided an analog integrating circuit which is wired to
integrate the pulses from the phase comparator. The counter is
bidirectional such that pulses from one channel will cause
integration in the opposite direction to pulses from the other
channel, see FIG. 6. Being an analog operation, the counting is
stored as a capacitor voltage. The pulse width will therefore
determine the increment in capacitor voltage per pulse in this
preferred configuration, but this is not an essential feature.
The pulse counting could be achieved by means of digital counters,
however, such a design is complex for two reasons. Firstly, it is
necessary to count both incrementally and decrementally. Secondly
it is highly desirable to permit the count value to "decay" over a
period of time, to overcome long term environmental changes which
would cause false alarms. These features are not difficult to
achieve digitally but are more costly than the analog method.
Also provided in the preferred embodiment of the counter is a
sensitivity control. This is a rheostat wired to determined the
count value for pulses, thereby affecting the number of pulses
required before the threshold limit of counting is reached and the
alarm circuitry consequently activated. This means that the amount
of movement in one direction required to cause an output activation
is adjustable. Thus, it is possible to adjust the sensitivity of
the system to provide immunity to a certain degree of environmental
movement. This overcomes the need for a pre-set sensitivity within
the counter which would be a compromise and may be excessive for
troublesome environments yet in-adequate for high-risk areas.
This feature of adjustable sensitivity is complex to achieve with a
digital counting circuit, in addition to the features of
bidirectional counting and slow decay.
* * * * *